1. Technical Field
The invention is directed to the application of adhesives. Particularly, the invention is directed to a system and process for spraying water-based adhesives, and an air gun for drying sprayed water-based adhesives.
2. Discussion
Adhesives are well-known for providing or promoting adhesion between two articles. Earliest adhesives were based on naturally-occurring substances with little or no processing from their natural forms. Many adhesives are still based on naturally-occurring substances, but have been subjected to processing.
The most notable development in adhesives in recent times has been the use of organic compound-based adhesives. These modern adhesives are based on the synthetic derivation of organic polymers. Solvent-based adhesives are used in the construction and manufacturing industries.
Typical of these types of adhesives is the "hot-melt" thermoplastic adhesive. Present-day versions of the hot-melt adhesive are composed of polymers such as ethylene-vinyl acetate copolymers, polyamides, polyesters, and polyethylene.
Hot-melt adhesives may be applied in a thick consistency or may be applied as a spray. Devices for spraying heated hot-melt adhesive materials are known. A typical example of such a device is disclosed in Ziecker et al U.S. Pat. No. Re. 33,481 which is directed to a spray system for spraying heated hot-melt adhesive in elongated strands or fibers in controlled spiral patterns.
Boger et al. U.S. Pat. No. 5,065,943 discloses a cap adapted for use with an adhesive dispensing device. Like the device of Ziecker et al., the Boger et al. device lays down an elongated adhesive fiber onto a substrate in a controlled spiral pattern.
While these devices have utility in providing a method for dispensing solvent-based adhesives, these adhesives themselves are regarded as being undesirable for widespread use. A popular solvent used in these adhesives has historically been trichlorofluoromethane (fluorotrichloromethane) derived from carbon tetrachloride and hydrogen fluoride. While providing good adhesion, trichlorofluoromethane is believed to be responsible in part for depletion of the ozone layer. As a result, methylene chloride (dichloromethane) has been more recently substituted for trichlorofluoromethane to overcome the ozone-depletion problem. However, methylene chloride is itself now believed to pose a danger as a carcinogen. It is now apparent that while solvent-based adhesives provide very good adhesion characteristics, their usefulness is severely compromised by their known and suspected dangers to people and the environment.
As a substitute for these adhesives, water-based adhesives have been more recently used. While overcoming the chemical dangers of solvent-based adhesives, the principal difficulty of these adhesives is that they dry very slowly when compared to their solvent-based counterparts, particularly when these adhesives are sprayed onto a substrate. Known systems for applying water-based adhesives that demonstrate an acceptable drying time are wanting.